Kotlin sieve using sequences

Question

While learning about functional programming the simple sieve example was brought up in Haskell. I wanted to try a Kotlin implementation using sequences.

Are there any other quick wins without modifying the structure too much?

import kotlin.coroutines.experimental.buildSequence
import kotlin.system.measureTimeMillis

fun sieve(isPrime: Int, ints: Sequence<Int>): Boolean = with(ints.first()){
    return when {
        isPrime  < 2 -> false
        isPrime == 2 -> true
        isPrime == this -> true
        isPrime.rem(2) == 0 -> false
//        isPrime.and(1) == 0 -> false // same way to check if number is even
        this > isPrime -> false
        else -> sieve(isPrime, ints.filter { n -> n.rem(this) != 0 })
    }
}

fun main(args: Array<String>) {
    val lazySeq = buildSequence { for (i in 3..Int.MAX_VALUE step 2) yield(i) }
    println("Duration = ${measureTimeMillis { println("isPrime(4057) = ${sieve(4057, lazySeq)}") }}ms")
//    (2..200).forEach { println("isPrime($it) = ${sieve(it, lazySeq)}") }
}

Answer 1

Here's my take on this:

import kotlin.system.measureTimeMillis

fun sieve(xs: Sequence<Int>): Sequence<Int> = sequence {
    val head = xs.first()
    val tail = xs.drop(1).filter { it % head != 0 }
    yield(head)
    for (i in sieve(tail))
        yield(i)
}

val primes = sieve(generateSequence(2) { it + 1 })
fun isPrime(n: Int) = primes.contains(n)

val durationMs = measureTimeMillis {
    println("isPrime(4057) = ${isPrime(4057)}")
}
println("Duration = $durationMs ms")

1. Start with the first element in the sequence - number 2
2. Filter out all numbers divisible by 2
3. Yield 2
4. Yield the rest recursively:
   1. Start with the first element in the sequence - number 3
   2. Filter out all numbers divisible by 3
   3. Yield 3
   4. Yield the rest recursively...